<!DOCTYPE html>
<html>
  <body>
    <span id="x" style="position: sticky; bottom: 75px;">
      <div></div>
    </span>
    <script>
      document.addEventListener("MozReftestInvalidate", function() {
        document.getElementById('x').style.bottom = '-3000px';
      });
    </script>
  </body>
</html>
